TUP_CWD uses Unix / separator char on Windows. If a Tupfile is in the root, or one dir deep, it's OK because TUP_CWD is just e.g. ... However, if the Tupfile is two or more dirs deep, then it is e.g. ../.. and makes an invalid command exe.

Alternatively/Additionally it could be that the mixed separator should be OK, but there is a problemat the exec/system() call.

/Tuprules.tup

THE_ROOT=$(TUP_CWD)

/foo/bar/Tupfile

include_rules

: |> $(THE_ROOT)\wvenv\Scripts\python --help > %o |> help.txt

[ tup ] [0.110s] Executing Commands...
* 0) foo\bar: ../..\wvenv\Scripts\python --help > help.txt
'..' is not recognized as an internal or external command,
operable program or batch file.
 *** tup messages ***
 *** Command ID=1432 failed with return value 1